This invention concerns a rectangular response optical filter for partitioning a limited spectral interval and having optical fibres, preferably single mode fibres, as input and output gates.
The evolution of wavelength multiplexed optical fibre telecommunications requires the development and the optimisation of such devices. It has been sought in particular to partition a wide spectrum into spectral domains, here called limited spectral intervals, while avoiding any superimpositions and cross-talk that might derive therefrom.
Numerous devices have already been proposed to that effect, whereas most of them consist in spreading the luminous spectrum in a plane and in placing in the said plane, a slot delineating the narrow spectral band that one wishes to select, but then the light cannot be recoupled efficiently in a monomode optical fibre.
Other devices implement sets of optically guided components: couplers, multiplexers-demultiplexers, . . .
The optimisation of such devices implies the provision of rectangular transmission functions and without loss in the limited spectral interval light flux selected, i.e. in a representation of the intensity of the light flux transmitted as a function of the wavelength as that on FIG. 1, the edges of the partitioned band should be as vertical as possible, the apex should be as flat as possible and the losses as little as possible. The apex can be flattened according to the state-of-the-art by generating losses.
We also know a document (I. Nishi and al., December 1987) that divulges a wide-band multiplexer-demultiplexer for multimode filter. It suggests the implementation of a retrodispersing system in Littrow configuration with respect to an input fibre and to output fibres. This document specifies that the width of the pass-band of such a device is determined by the length of the retroreflector.
Besides, in a published article (Chi-Luen Wang and al., 1994), is described an external cavity laser wherein the external cavity is set up so that it enables filtering of two wavelengths. Filtering is performed by reflecting bands forming reflecting mirrors cooperating with a grating.
The implementation of the teachings of these documents does not enable to realise a transmission-stable device and ensuring good accuracy.
The inventors have set themselves the target of providing such a device implementing a grating-reflector assembly in Littman-Metcalf configuration in order to take advantage of the high performances offered by such a type of configuration and that such a device does not generate any losses and possesses optical fibres, preferably monomode as input and output gates, ensuring optimized stability and accuracy.
Thus, the invention concerns a rectangular response optical filter for patitioning a limited spectral interval in a wide spectrum light flux comprising:
an input optical fibre having one end,
a grating-reflector assembly in Littman-Metcalf configuration,
a converging collimation optical system at whose focal point is located the end of the input fibre,
a converging focusing optical system placed between the grating and the reflector,
at least one reflector placed in the focal plane of the focusing optical system whose dimension is limited in the dispersion plane, whereas the position and limited dimension of the reflector in the dispersion plane determine the partitioned spectral interval.
According to the invention, the optical filter comprises a polarisation separator placed between the input fibre and the grating and generating two elementary light beams parallel and polarised orthogonally with respect to one another, whereas a plate xcex/2 is placed on one of the elementary beams in order to generate two elementary parallel beams polarised in a direction perpendicular to the lines of the grating, whereas the reflector of Littman-Metcalf configuration is sending each elementary beam back to path and in opposite direction in relation to one another.
In different embodiments each exhibiting its own specific advantages and liable to be used in the compatible technically combinations:
the input optical fibre is a monomode fibre,
the light flux generated with limited spectrum is collected in an output optical fibre distinct from the input fibre and of the same type as the latter,
the optical filter comprises several optical output fibres, each connected to a reflector, whereas these reflectors are positioned in the focal plane of the focusing optical system and have a small dimension in the dispersion plane while determining a particular spectral interval,
the light flux generated with limited spectrum is collected by the input fibre and the latter carries an optical circulator enabling to separate the output flux from the incoming flux without any energy loss,
the optical filter comprises a folding reflector doubling the number of passages of the light beam on the grating,
the reflector of Littman-Metcalf configuration is a planar mirror connected to a bi-prism,
the reflector of Littman-Metcalf configuration is a truncated dihedron.